Purification of aqueous solutions of organic acids

ABSTRACT

The invention relates to a method and a device for the purification of an aqueous solution of an organic acid having a boiling point at atmospheric pressure of less than 450° C., which further has approximately 275 g carboxylate ions/l or less, preferably 250 g carboxylate ions/l or less, and preferably less than 1% by weight ionic impurities, calculated on the basis of the total solution. In particular, the invention relates to a method and a device for the continuous purification and concentration, on an industrial scale, of an aqueous solution of an organic acid having a boiling point at atmospheric pressure of less than 450° C. According to the method, this solution is subjected to two or more distillation steps, the first distillation step being carried out at a temperature of from 80° to 150° C. and a pressure of from 50 to 250 mbar, and the second distillation step being carried out at a temperature of from 80° to 200° C. and a pressure of from 0.01 to 50 mbar.

This application is a continuation application of U.S. Ser. No.10/130,990, filed Oct. 1, 2002, now U.S. Patent No. 7,410,556, which isa national phase application under 35 U.S.C. § 371 of PCT/NL2000/000865,filed Nov. 27, 2000, which claims priority from NL application number1013682, filed Nov. 26, 1999.

SUMMARY OF THE INVENTION

The invention relates to a method and a device for the purification ofan aqueous solution of an organic acid having a boiling point atatmospheric pressure of less than 450° C., which comprises approximately275 g carboxylate ions/l or less, preferably 250 g carboxylate ions/l orless, and preferably less than 1% by weight ionic impurities, calculatedon the basis of the total solution, and in particular to a method and adevice for the continuous purification and concentration, on anindustrial scale, of an aqueous solution of an organic acid having aboiling point at atmospheric pressure of less than 450° C. The methodand the device according this invention can also be employed for theproduction and purification of cyclic dimers of said organic acids,provided that said cyclic dimers have a boiling point at atmosphericpressure of less than 450° C.

In the context of the invention, the organic acid includesmonocarboxylic, dicarboxylic and polycarboxylic acids. Moreover, theterm organic acid is to be construed as comprising the pure acid as wellas a mixture of substantially pure acid and optionally a small amount ofoligomerized or polymerized organic acid and/or cyclic dimers of theorganic acid. Examples of these organic acids are citric acid, glycolicacid, acetic acid, acrylic acid, methacrylic acid etc. The organic acidis preferably an α-hydroxy acid and in particular lactic acid. Likewise,the terms α-hydroxy acid and lactic acid are understood as meaning amixture of substantially unpolymerized α-hydroxy acid or unpolymerizedlactic acid and possibly a small amount of polymerized α-hydroxy acidand the cyclic form of the dimer, or lactic acid and dilactide (thecyclic form of the dimer).

THE PRIOR ART

WO 98/55442 describes a method for the purification of laxtic acid ofthis type in which a solution of lactic acid in water as is obtainedfrom fermentation or a different source is subjected to at least threesteps. The first step comprises the removal of ionic substances whichmay catalyse the oligomerization of lactic acid from the aqueoussolution of lactic acid, the solution containing less than 80%,preferably less than 50%, and in particular less than 30% lactic acid.Preferably, a cation exchanger is used to remove cationic substances,and then an anion exchanger is used to remove anionic substances. Thesecond step comprises concentrating the solution to a concentration of50 to 90%, preferably 70 to 90%, by evaporation under reduced pressure,the pressure being 50 to 500 mbar and preferably 50 to 250 mbar and thetemperature being kept as low as possible. The evaporation is preferablycarried out with the aid of flowing film evaporation. The third stepcomprises a distillation at a pressure of from 0.001 to 100 mbar,preferably 0.1 to 20 mbar, and in particular 1 to 10 mbar, thetemperature of the wall of the evaporation device being 80° to 160° C.and preferably 110° to 160° C. The distillation is preferably carriedout with the aid of a mechanically moved thin film evaporator or a shortpath evaporator and provides pure lactic acid. If appropriate, apost-concentration step may be carried out between the second and thirdsteps. This post-concentration is also preferably carried out using amechanically moved thin film evaporator or a short path evaporator, at apressure of from 10 to 500 mbar, preferably 50° to 250 mbar, and at atemperature of from 50° to 150° C., preferably from 80 to 120° C. It isclaimed that in this post-concentration the concentration of thesolution as obtained from the first concentration step (step two) can beraised to 100% lactic acid.

One drawback of this method is that in the first concentration step(step two) the separation between the solution containing concentratedlactic acid and the fraction which contains substantially water is poor,and consequently the fraction which contains substantially water alsocontains a significant amount of lactic acid and impurities which has anadverse effect on the yield of the method. Moreover, thiswater-containing fraction has to be cleaned before it can be dischargedor recycled into the method. Another drawback of this method is thesudden fall in pressure (from the first to the second concentrationstep). This disturbance to the liquid phase reduces the quality in thesecond concentration step, particularly if the concentrate from thefirst concentration step contains dissolved gases or water. Adisadvantage of a short path evaporator is that during operationsplashing of the residual product against the internal condenser occurs.

Other methods for the purification of organic carboxylic acids such aslactic acid are described in French patent specification 1.290.212 andU.S. Pat. No. 1,594,843. The French patent specification describes theuse of ion exchangers for removing cationic substances. U.S. Pat. No.1,594,843 describes the purification of an aqueous solution of lacticacid, during which a thin film evaporator is used, operated under avacuum of 27 inches of mercury or greater and a temperature of 450° to600° F. These conditions correspond to a pressure of approximately 436mbar or lower and a temperature of approximately 232° to 316° C. Thesemethods provide lactic acid of insufficient purity.

Example 1 of Belgium patent 9400242 describes the short pathdistillation of lactic acid on a laboratory scale (the device used is alab scale device with an evaporation and condensation surface area of0.06 m² and a throughput of approximately 0.04 to 5 kg/h). The pressurewas varied between 50 and 130 mbar, the temperature of the film wasvaried between 1000 and 125° C., and the flow rate was varied between325 and 1150 g/h. The yield varied from 62 to 95%, the highest yieldhaving been obtained at a pressure of 50 mbar, a film temperature of100° C. and a flow rate of 1150 g/h.

DETAILED DESCRIPTION OF THE INVENTION

The object of the present invention is to resolve the drawbacks of themethods according to the prior art. The present invention thereforerelates to a method for the purification of an aqueous solution of anorganic acid having a boiling point at atmospheric pressure of less than450° C., preferably an α-hydroxy acid and in particular lactic acid,which comprises approximately 275 g carboxylate ions/l or less,preferably 250 g carboxylate ions/l or less, and preferably less than 1%by weight, preferably less than 0.1% by weight, and in particular 0.001to 0.01% by weight ionic impurities, calculated on the basis of thetotal solution, the solution being subjected to two or more distillationsteps. This method can also be used for the production and purificationof cyclic dimers of the organic acid, provided that the boiling point ofthe dimer is less than 450° C. The organic acid is preferably anα-hydroxy acid and in particular lactic acid. One advantage of thismethod is that in the first distillation step, during which the organicacid concentration is increased to at least 95% by weight organic acid,calculated on the basis of the weight of the product of this firstdistillation step, an aqueous fraction is obtained which contains lessthan 1% by weight organic acid, so that this aqueous fraction can bedischarged or recycled into the method without further cleaning.

According to the invention, the aqueous solution is preferably distilledat reduced pressure in a first distillation step at a temperature offrom 80° to 150° C., in particular from 100° to 140° C., and at apressure of from 50 to 250 mbar, in particular from 60 to 150 mbar.However, if the pressure is lower than 50 mbar, it is necessary to usecooling water at a temperature of lower than ambient temperature (25°C.) in order to condense the water vapour released, which has an adverseeffect on the process management and process economy.

In the first distillation step, the aqueous solution is brought into thevapour phase by means of film evaporation. This film evaporation can beachieved using one or more falling film evaporators, wiped filmevaporators and/or thin film evaporators. Evaporation devices of thistype are known in the prior art and are described, for example, inKirk-Othmer, Encyclopaedia of Chemical Technology, fourth edition,Volume 9, pp. 959-981 (1994). Then, the vapour is preferably passed to afirst distillation column, in which separation into two fractions iscarried out under reflux conditions. In this way, a first top fractionis obtained, which substantially comprises water and contains no morethan 1% by weight, preferably no more than 0.1% by weight organic acid,calculated on the basis of the weight of the top fraction, and a firstbottom fraction (product) which contains at least 95% by weight,preferably 99 to 99.9% by weight Total Organic Acid (TOA), calculated onthe basis of the weight of the bottom fraction. It will be clear to theperson skilled in the art that it is desirable for the first bottomfraction to contain the maximum possible amount of Total Organic Acid.Therefore, the first bottom fraction may consist of 100% by weightorganic acid. It will also be clear to the person skilled in the artthat a plurality of distillation columns can be used in the firstdistillation step.

Total Organic Acid (TOA) content is de acid content after hydrolysis ofintermolecular ester bonds by using excess base and is determined bybacktitration with acid. The Total Organic Acid content gives thereforethe amount of monomeric acid, (cyclic and/or linear) dimeric acid,oligomeric acid and polymeric acid. The Free Organic Acid (FOA) contentis determined by a direct titration with a base, i.e. prior to thehydrolysis of the intermolecular ester bonds. The content of MonomericOrganic Acid (MOA) is here defined as:MOA=TOA−2×(TOA−FOA)provided that TOA−FOA<10%.

The first distillation column which is used in the first step preferablyhas a plate number of from 0.1 to 10, preferably 1 to 10 and inparticular 1 to 5.

The product of the first distillation step is then distilled in a seconddistillation step, preferably under vacuum, forming a top fraction,which contains at least 99.5% by weight pure Total Organic Acid, and adistillation residue (bottom fraction). In the context of the invention,under vacuum is to be understood as meaning a pressure of from 0.01 to50 mbar, more preferably 0.1 to 20 mbar, in particular 1 to 10, and morein particular 2 to 10 mbar. In this second step, the temperature ispreferably 80° to 200° C., more preferably 100° to 200° C., inparticular 100° C. to 140° C. and more in particular 110° to 140° C. Thesecond distillation step is preferably carried out in one or more shortpath distillation devices, but in particular in one or more vacuumdistillation units which do not have the disadvantage of splashing asoutlined above.

According to a more preferred embodiment of the invention, in the seconddistillation step the product from the first distillation step isbrought into the vapour phase by means of film evaporation, and then thevapour is passed to a second distillation column, separation into twofractions being brought about under reflux conditions. In this morepreferred embodiment, the product (the first bottom fraction) from thefirst distillation step is brought into the vapour phase preferably byusing one or more falling film evaporators, wiped film evaporatorsand/or thin film evaporators. The second distillation column preferablyhas a plate number of from 0.1 to 10, more preferably 1 to 10, and inparticular from 1 to 5. In this way, a second top fraction (product) isobtained, which substantially comprises the organic acid and whichcontains at least 99.5% by weight Total Organic Acid. The residue (thesecond bottom fraction) of this second distillation step containssubstantially sugars and products derived therefrom, some monomericorganic acid, remainder dimers, trimers and oligomers of the organicacid, cyclic dimers of the organic acid (hence, dilactide when theorganic acid is lactic acid) and other non-volatile components. It willbe clear to the person skilled in the art that a plurality ofdistillation columns can be used in the second distillation step. Thismore preferred embodiment of the second distillation step is preferablycarried out under a pressure and at a temperature which are the same asthose indicated above where one or more short path distillation devicescan be used. If very pure organic acid is desired, the top fractionobtained using this more preferred embodiment can be subjected to adistillation under vacuum as described above.

According to the invention, it is preferable for the product from thefirst distillation step (the first bottom fraction) to be subjected to aconditioning step (so-called “preflash”) before it undergoes a seconddistillation step, the pressure in this conditioning step preferablybeing the same as that used in the second distillation. However, thepressure may also be higher i.e. at most 50 mbar (the lowest pressurethat can be used in the first distilling step) and preferably at most 20mbar, and at least 10 mbar (the highest pressure that can be used in thesecond distillation step). This preferred embodiment has the advantagethat a residual quantity of water and dissolved gases are removed beforethe product is subjected to the second distillation step. For example,the product from the first distillation step may contain, for example96% by weight lactic acid, the use of a preflash allowing the lacticacid content to be increased to, for example 99% by weight, so that inthe second distillation step it is possible to obtain purer lactic acidand to achieve more stable operation. The fraction of water which isremoved using a preflash generally contains 10 to 50%, preferably 10 to20% by weight Total Organic Acid.

The residue of the second distillation step can be recycled into themethod, purged, or processed in the same method but on a smaller scale(this requires a second distillation section), but is preferably firstsubjected to a depolymerization step, in particular because the residuecontains approximately 1% by weight Total Organic Acid, non-volatileremainder dimer, oligomers and polymers of the acid and the cyclisizedform of the dimer (e.g. dilactide when the acid is lactic acid),calculated on the basis of the total residue, and sugars. It will beobvious to the person skilled in the art that such a depolymerizationstep can me omitted if the organic acid does not contain reactive groupsthat may give rise to dimerization, oligomerization of polymerizationsuch as is the case when the organic acid is acetic acid.

The depolymerization step is preferably carried out by heating a mixturecomprising 30 to 70% by weight, preferably 40 to 60% by weight, of anaqueous stream which preferably contains 80 to 100% by weight water, and70 to 30% by weight, preferably 60 to 40% by weight, of the residue ofthe second distillation step at a temperature of from 60° to 100° C. forfrom 1 to 30, preferably 1 to 10 hours under atmospheric pressure. Thisaqueous stream is preferably taken from the first distillation stepand/or the preflash. The product from the depolymerization step ispreferably recycled to the first distillation step, is purged or issubjected to a similar distillation operation in a separate section on asmaller scale.

Using the method according to the invention it is for example possibleto obtain lactic acid, which, after it has been heated for 2 hours underreflux, has a color of no more than 60 APHA units, preferably no morethan 40 APHA units, and in particular no more than 5 APHA units. TheseAPHA values are also attainable for other organic acids.

The invention will be further elaborated on the basis of the followingFigures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a flow scheme of the process according to the invention.

FIG. 2 shows diagrammatically a falling film evaporator.

FIG. 3 shows diagrammatically a wiped film evaporator.

FIG. 4 shows diagrammatically a short path evaporator.

FIGS. 5A and 5B show diagrammatically a vacuum distillation unitaccording to the most preferred embodiments of the invention.

FIGS. 6A-6D show flow diagrams of preferred embodiments of the firstdistillation step.

FIG. 7 shows diagrammatically a first distillation step coupled to avacuum distillation unit according to a preferred embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a preferred embodiment of the method according to theinvention. In this embodiment, a stream [1] of an aqueous solution of anorganic acid having a boiling point at atmospheric pressure of less than450° C., said steam [1] containing about 275 carboxylate ions/l or less,is subjected to a first distillation step so as to form a bottomfraction [2] which substantially contains the organic acid and a topfraction [3] which substantially contains water. The bottom fraction [2]is preferably subjected to a “preflash” so as to form a top fraction [4]and a bottom fraction [5], the top fraction [4] substantially comprisingwater and the bottom fraction [5] substantially comprising the organicacid. The bottom fraction [5] is then subjected to a second distillationstep, so as to form the top fraction [6] which substantially or entirelycomprises the pure organic acid, and a bottom fraction (residue) [7].This bottom fraction [7] is preferably subjected to a depolymerizationstep, in which an aqueous stream [10] is supplied which is preferablycomposed of the top fractions [3] and [4] and, if appropriate,additional water. The residue of the depolymerization step, stream [9],is discharged, and the product of the depolymerization step, stream[11], is further processed in one of the following ways:

-   1. purged [12],-   2. processed in a separate distillation section as described above    (hence, it comprises also two distillation steps and preferably also    a preflah), although on a smaller scale [13], or    1. Recycled to the First Distillation Step [14].

The invention also relates to a device for the purification of anaqueous solution of an organic acid having a boiling point atatmospheric pressure of less than 450° C. which comprises approximately275 g/carboxylate ions/l or less, preferably 250 g carboxylate ions/l orless, and preferably less than 1% by weight ionic impurities, calculatedon the basis of the total solution, the device comprising a firstdistillation device and a second distillation device.

According to a first preferred embodiment of the invention, a firstdistillation device comprises one or more film evaporation devices, suchas falling film evaporators, wiped film evaporators and/or thin filmevaporators, and one or more distillation columns, the distillationcolumns having a plate number of from 0.1 to 10, preferably 1 to 10, andin particular 1 to 5. The second distillation device preferablycomprises a device which operates under vacuum (i.e. a device which canbe operated at a pressure of from 0.01 to 50 mbar, more preferably 0.1to 20 mbar, and in particular 1 to 10 mbar and more in particular 2 to10 mbar), and this device comprises one or more short path evaporatorsor vacuum distillation units, preferably vacuum distillation units.

According to a second, but more preferred embodiment of the invention,the first and second distillation devices comprise one or more filmevaporation devices, and one or more distillation columns, in which casethe film evaporation devices may be falling film evaporators, wiped filmevaporators and/or thin film evaporators, and in which case thedistillation columns have a plate number of from 0.1 to 10, preferably 1to 10, and in particular 1 to 5.

FIG. 2 diagrammatically depicts a falling film evaporator (an externalcondenser device is usually present, but it is not shown). A device ofthis type comprises a vessel [20] in which there are heater elements[21] through which a heating medium is passed via inlet and outletopenings [22] and [23]. The liquid which is to be evaporated is passedinto the evaporator via inlet opening [24], the top fraction [25]containing the most highly volatile component(s), and the bottomfraction [26] containing the least volatile component(s). The topfraction is discharged to the external condenser device via vacuum line[27]. Any residue can be removed via outlet opening [28]. Drawbacks offalling film evaporators are that the laminar films which flow downwardsalong the heater elements give rise to so-called “hot spots”, with theresult that the product to be purified becomes overheated and is thendecomposed. In the present case, “hot spots” of this type may lead todecomposition of the organic acid to thermal decomposition products andoligomerization of the organic acid. Other drawbacks of evaporators ofthis type is that the considerable temperature differences which prevailin the vessel do not allow optimum distillation results to be achievedand that the pressure in evaporators of this type cannot be lower than20 mbar on account of the pressure drop which is caused by vapours whichflow from the surface of the heater elements towards the externalcondenser device.

FIG. 3 diagrammatically depicts a wiped film evaporator. A device ofthis type comprises a vessel [30] and a heating jacket [31] throughwhich a heating medium is passed via inlet and outlet openings [32] and[33]. An agitator [34] which is provided with paddles, wipers orscrapers, is arranged in the vessel, so that the liquid which is to beevaporated and is passed into the device via inlet opening [35] isdispersed over the wall of the heating jacket of the device as a film.The most highly volatile component(s) leave(s) the device via vacuumline [36] to an external condenser device, and the least volatilecomponent(s) leave(s) via outlet opening [37]. The most significantdrawback of wiped film evaporators is that the operating pressure windowis minimal (only a few mbar), on account of the pressure drop of vapourswhich flow from the surface of the heater jacket to the externalcondenser device. Another drawback is that as the internal temperaturefalls, the pressure has to be reduced, leading to a considerableincrease in the vapour volume and high flow resistances. The latterdrawback can be overcome if an external condenser device is replaced byan internal condenser device which is arranged at a short distance fromthe wall of the heater jacket, so that in fact a short path evaporatoris obtained.

A diagrammatic illustration of a short path evaporator is shown in FIG.4. An evaporator of this type comprises a vessel [40], a heating jacket[41], a roller wiper system [42], which is driven by an external motor,for applying a film of the liquid which is to be evaporated, and aninternal condenser device [43]. A heating medium is passed through theheater shell via inlet and outlet openings [441] and [442], whilecooling water for the condenser device is passed through inlet andoutlet openings [451] and [452]. The liquid which is to be evaporated issupplied to the evaporator via inlet opening [46]. The most highlyvolatile components are discharged via vacuum line [47], and the productis discharged via outlet line [48]. Advantages of a short pathevaporator are in particular efficient heat transfer and the avoidanceof “hot spots”, the continuous operation of the distillation with ashort residence time of the product which is to be purified on the wallof the heating jacket, and a working pressure which may be 0.01 bar.However, short path evaporators have the disadvantage that duringoperation splashing of the distilled product occurs against the internalcondenser.

FIGS. 5A and 5B show vacuum distillation units according to the mostpreferred embodiments of the invention. These vacuum distillation unitscomprise a vessel [50, 60] for the evaporation of a film of the productwhich is to be evaporated, said vessel being provided with a heatingjacket [51, 61], a feed inlet line [56, 66], a residue outlet line [57,67], and a wiper system for providing a film of the product to beevaporated (not shown). The vapour is passed to a distillation column[52, 62]. The distillation column is provided with a packaging [53, 63],a cooling device [54, 64], a vacuum line [55, 65] and optionally adistribution plate (not shown) upstream of the packaging.

The distillation column preferably has a plate number of from 0.1 to 10,more preferably of 1 to 10 and in particular from 1 to 5. Vessel [50,60] is preferably a falling film evaporator, a wiped film evaporator ora thin film evaporator, e.g. such as shown in FIGS. 2 and 3. Refluxconditions prevail in the distillation column [52, 62], so that optimumseparation is brought about by means of distillation. This reflux isachieved by recycling product coming out of the product line [581, 68]via recycle line [59, 69] to the distillation column. The bottom productfrom the distillation column [57, 67], that in the present case containsthe organic acid and impurities, is preferably recycled to the device[50, 60; not shown]. The top product [581, 581, 68] containssubstantially pure organic acid. In the apparatus according to FIG. 5A,the vapour is separated into a less pure product (which leaves thedistillation column via line [582]) and a more pure product (whichleaves the distillation column via line [581]).

It has been found that the second distillation step can be more suitablecarried out using a vacuum distillation unit as shown in FIGS. 5A and 5Binstead of a short path distillation device.

FIGS. 6A-6D show preferred embodiments of the first distillation step,wherein 6A shows the least preferred embodiment and 6C and 6D show themost preferred embodiments. As is outlined above, the first distillationstep is conducted in a vessel [71] for evaporation of the product to beevaporated, i.e. a wiped film evaporater, a thin film evaporator or afalling film evaporator, and an external cooling device [72]. The feedenters the evaporator via feed inlet line [73] whereas the bottomsleaves the evaporator via bottoms exit line [75] and the evaporatedproduct via product exit line [74]. The evaporated product is condensedin cooling device [72] and liquid product leaves the distillation unitvia line [76]. As shown in FIG. 6B, part of the liquid product can berecycled to the evaporator via line [77] to maintain a reflux which, asis known by the skilled in the art, improves the separation. In FIG. 6C,the feed is introduced in distillation column via line [73], saiddistillation column being in open contact with evaporator [71] via line[74]. Due to the reflux conditions prevailing within the distillationcolumn and within the evaporator (part or all of the bottoms leaving thedistillation column via line [80] is recycled to the evaporator via line[79]), separation is established within the combinationevaporator/distillation column. In FIG. 6D a modification of theconfiguration of FIG. 6C is shown, wherein the most importantmodification is that the feed is introduced into the evaporator insteadof into the distillation column.

According to the invention, an aqueous solution of an organic acidhaving a boiling point at atmospheric pressure of less than 450° C. istherefore preferably purified in the following way:

-   (A) a first distillation step in which one or more falling film    evaporators, wiped film evaporators and/or thin film evaporators are    used,-   (B) a “preflash”,-   (C) a second distillation device in which one or more short path    devices and/or vacuum distillation units is/are used, and-   (D) a depolymerization step (if the organic acid can dimerize,    oligomerize and/or polymerize).

More preferably, according to the invention the aqueous solution of theorganic acid is purified in the following way (best mode of theinvention):

-   (A) a first distillation step in which one or more falling film    evaporators, wiped film evaporators, and/or thin film evaporators    and one or more distillation columns are used,-   (B) a “preflash”,-   (C) a second distillation device in which one or more short path    distillation devices and/or vacuum distillation units is/are used,    and-   (D) a depolymerization step (if the organic acid can dimerize,    oligomerize and/or polymerize).

In particular, according to the invention an aqueous solution of lacticacid is purified in the following way:

-   (A) a first distillation step in which one or more falling film    evaporators, wiped film evaporators and/or thin film evaporators and    one or more distillation columns are used,-   (B) a “preflash”,-   (C) a second distillation device in which one or more vacuum    distillation units is/are used, and-   (D) a depolymerization step (if the organic acid can dimerize,    oligomerize and/or polymerize).

EXAMPLES Colour Determination of Lactic Acid

The method is based on ASTM D 5386-93. The colour of lactic acid isvisually established by using a series of standard APHA solutions.Instead, the colour can be determined spectrofotometrically.

The standard solutions are prepared as follows. An amount of 1.245 gK₂PtCl₆ and 1.000 g of CoCl₂.6H₂O are dissolved in purified water.Subsequently, 100 ml 37% HCl is added and the solution obtained isdiluted to 1000 ml by adding purified water. This stock solution (500APHA units) has to meet the following specifications:

Wave length (nm) Absorbance 430 0.110-0.120 455 0.130-0.145 4800.105-0.120 510 0.055-0.065

Thereafter, standard solutions are made of 5-300 APHA units with aninterval of 5 APHA units by pipetting 1.0-60.0 ml of the stock solutionin 100 ml volumetric flasks. Subsequently, the flasks are made up to 100ml with purified water. When new solutions have to be made, the sameflask must be used for the preparation of the same solution.

In the visual method, the colour of the lactic acid sample is determinedabove a light source using a 100 ml calorimetric tube where the standardAPHA solutions are used as reference. In the spectrofotometric method, abase line is first established whereafter a calibration follows. Then,the aborbance of the lactic acid sample is determined and the APHA valueis determined from the calibration table.

The Effect of Splashing

Samples taken from a commercial plant that was either equipped with ashort path evaporator (SPE) or a vacuum distillation unit (VDU)according to the invention were analysed for their colour. Not only thecolour of the neat sample was determined, but also after heating thesample for about two hours under reflux. The results are shown in TableI.

TABLE I Split Colour (APHA) Distillate Residue No heating After heatingVDU 80 20 20 34 SPE 75 25 40 60

The data show that at under almost identical operation conditions theVDU afforded a better product quality in terms of colour than the SPE

The samples and the feed were further analysed for their Ca²⁺ content.Ca²⁺ was used as the parameter to determine splashing since it does notevaporate under the conditions of the method according to the invention.The splashing factor was calculated according to the following formula:% splashing={[Ca²⁺]_(distillate)/[Ca²⁺]_(feed)}*100*0.75wherein the number 0.75 stands for the split. The data are shown belowin Table II. These data show that avoiding splashing (by using a VDUinstead of a SPE) has a great advantageous effect on the productquality.

TABLE II [Ca²⁺]_(feed) [Ca²⁺]_(distillate) (ppm) (ppm) % Splashing VDU(with 90/10 split) 2180 — (a) 0.00 1730 — (a) 0.00 2440 — (a) 0.00 5900— (a) 0.00 6172 — (a) 0.00 6370 0.20 0.00 3236 — (a) 0.00 3840 — (a)0.00 14570 0.38 0.00 6245 — (a) 0.00 6532 — (a) 0.00 23720 — (a) 0.006402 — (a) 0.00 5195 — (a) 0.00 5510 — (a) 0.00 0.00 (av.) SPE (with75/25 split) 2749 36 0.98 (a) the amount was below the detection limitof 0.03 ppm.

1. A purification device comprising a first distillation device operablyconnected to a second distillation device, wherein the firstdistillation device comprises a first film evaporator and a firstdistillation column, wherein a vapor phase outlet of the first filmevaporator is connected to the first distillation column, wherein abottom fraction outlet of the first film evaporator is connected to thesecond distillation device, and wherein the second distillation devicecomprises a second film evaporator and a second distillation column. 2.The device according to claim 1, wherein the first and second filmevaporators are a wiped film evaporator, a thin film evaporator, afalling film evaporator, or a combination thereof.
 3. The deviceaccording to claim 1, wherein the first and second distillation columnshave a plate number of from 0.1 to
 10. 4. The device according to claim1 further comprising a preflash device operably connecting the first andsecond distillation devices.
 5. The purification device according toclaim 1 further comprising a depolymerization device operably linked tothe second distillation device.
 6. A purification device comprising afirst distillation device operably connected to a second distillationdevice and a depolymerization device operably linked to the seconddistillation device, wherein the first distillation device comprises afilm evaporator and a distillation column, wherein a vapor phase outletof the film evaporator is connected to the distillation column, andwherein a bottom fraction outlet of the film evaporator is connected tothe second distillation device.